An electric sleeve heater, in particular for heating a spray nozzle of an injection-molding or die-casting machine, in particular for plastic extruders, has a tubular casing of metal that is fitted over the part to be heated and in which an electric heating element is imbedded, as well as a thermocouple as a temperature sensor whose sensor tip is positioned at an end of the tubular casing such that it is in heat-conducting contact with the outer surface of the part being heated.
A corresponding electric sleeve heater is known from US 2010/0092595 (DE 20 2006 018 576). Here, a radially through going hole in the wall of the tubular casing is provided near and end region, in or through which through a free end of the temperature sensor or a portion near the free end of the temperature sensor, is guided. The part being heated is formed by a steel tube, while the tubular casing can be produced from copper or a copper alloy.
Since in this embodiment there is a spacing between the sensor tip and the outer surface of the part being heated, this free end of the temperature sensor, that is, the sensor tip, can be fixed by a holding element that when installed ensures contact between the free end of the temperature sensor and the part being heated. The holding element should thereby be a clamp made of temperature-resistant spring steel.
An embodiment of this type is disadvantageous in that there is a direct heat-conducting connection between the sensor tip of the thermocouple and the electrical heater. This direct contact is formed in part by the outer surface of the tubular casing of metal or also by the additionally provided metallic spring. As a result, this means that the temperature that the sensor tip of the thermocouple measures is not exactly the temperature of the part being heated in the region of the sensor tip. Instead, this measured value is distorted by heat conduction between the tubular casing of metal and the sensor tip or between the tubular casing of metal, the metal spring and the sensor tip.
An embodiment is known from DE 20 2008 013 626, in which the sensor tip of the thermocouple is fixed in heat-transmitting manner in a groove of a wall of the tube and is separated from the tube wall laterally by front-side indentations, that is, from the tubular casing of metal.
Here there is still direct heat-conducting contact between the tube wall section and the tubular casing, so that this again results in a distortion of the measured value that is recorded by the temperature sensor with the sensor tip on the outer surface of the part to be measured.
An arrangement is also hereby provided in which in addition a spring steel sheet is to be provided in order to press the exposed tube wall section against the part being heated and likewise to press the tip of the thermocouple against the part being heated. Here, too, there is again a heat-conducting connection between the tubular casing of metal and the sensor tip of the thermocouple, so that distortions of the measured values again result from this.